Local Hormones : act locally on neighbouring cells or on the same cell that secreted them without first entering the bloodstream.
Paracrines : local hormones that act on neighbouring cells.
Autocrines : local hormones that act on the same cell that secreted them.
Local hormones are usually inactivated quickly. Circulating hormones tend to linger but in time they are inactivated by the liver and excreted by the kidneys. In cases of liver or kidney failure, excess of specific hormones may be evident.
Chemical Classes of Hormones
Hormones are chemically divided into 2 broad classes :
1) Lipid-Soluble Hormones
Steroid Hormones : derived from cholesterol and have a large diversity of functions
Thyroid Hormones: are synthesized by attaching iodine to the amino acid tyrosine.
Nitric Oxide (NO): is both a hormone and a neurotransmitter.
2) Water-Soluble Hormones
Amine Hormones : are synthesized by decarboxylating (removing a CO2 molecule) certain amino acids. Examples include: catecholamines such as epinephrine, norepinephrine, and dopamine (synthesized by modifying the AA tyrosine); Histamine (synthesized by modifying the AA histidine); Serotonin and Melatonin are derived from tryptophan. Peptide Hormones & Protein Hormones : smaller peptide hormones are chains of 3-49 AAs; larger protein hormones include 50-200 AAs. Examples include antidiuretic hormone and oxytocin; protein hormones include eGH (Equine Growth Hormone) and Insulin. Several of the protein hormones such a Thyroid-Stimulating Hormone have attached carbohydrate groups and thus are Glyocoprotein Hormones.
Eicosanoid Hormones : derived from arachidonic acid and have 2 major types (from chapter 2)
Prostoglandins which modify responses to hormones, contribute to inflammation, prevent stomach ulcers, dilate airways to lungs, regulate body temperature, influence the formation of blood cells. Leukotrienes: participate in allergy and inflammatory response.
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